The cognitive processing components underlying performance on the Digit Symbol Test are investigated. Hypothesized cognitive components are digit encoding, response selection, symbol encoding, search, and memory. The main goal is to determine what the Digit Symbol Test measures in terms of cognitive processing. Other goals are to describe correlations between performance on the Digit Symbol Test and other tests of verbal ability and perceptual speed, investigate developmental changes in the processes underlying Digit Symbol performance, and examine sex differences in terms of the hypothesized cognitive components. The primary long-term objective is to develop measures of intelligence that not only predict performance, but that also provide guidance in the development of strategies for improving performance. Subjects are children, between the ages of 9 and 11 years; young adults, between the ages of 18 to 25 years; and elderly adults, 65 years and over.